When is a car using more fuel?
Discussion
Talking about a small N/A engine here to keep it simple.
I commute to work over some pretty large gradients and always wonder the best way to tackle them to save fuel.
Is the car using more fuel when dropping into 2nd and booting it up the hill or keeping it in second, foot to the floor and not really making any progress?
Also, would a car use more fuel if you slowly (at say, half throttle) revved to 5k before changing or floored it to say 2.5k and then changed?
No real point to this thread, just something I've thought about a number of times.
I commute to work over some pretty large gradients and always wonder the best way to tackle them to save fuel.
Is the car using more fuel when dropping into 2nd and booting it up the hill or keeping it in second, foot to the floor and not really making any progress?
Also, would a car use more fuel if you slowly (at say, half throttle) revved to 5k before changing or floored it to say 2.5k and then changed?
No real point to this thread, just something I've thought about a number of times.
The closest easy approximation is as above, you want it in the highest gear in which the engine "feels happy" with the slight caveat that you mustn't open the throttle beyond about 80-90% because that apparently causes modern cars to over-fuel and hence burn less efficiently.
There are a few other oddities. Modern petrol (and presumably diesel) engines shut fuel off completely at above about 2000rpm if you are fully off the throttle (if you engine brake down from about 2500rpm to 1500rpm in a low gear you can usually feel the transition as it starts to fuel again), so if it's possible to engine brake down hills without losing speed with the revs above that point, you should do so and you will use no fuel at all. If you can't do that, the most efficient thing to do is often to coast, which isn't a very good idea from a car control point of view. Some people say that it also affects the ability of brake servos, power steering, etc. to operate, but I've always been slightly dubious of this as long as the engine is running.
There are a few other oddities. Modern petrol (and presumably diesel) engines shut fuel off completely at above about 2000rpm if you are fully off the throttle (if you engine brake down from about 2500rpm to 1500rpm in a low gear you can usually feel the transition as it starts to fuel again), so if it's possible to engine brake down hills without losing speed with the revs above that point, you should do so and you will use no fuel at all. If you can't do that, the most efficient thing to do is often to coast, which isn't a very good idea from a car control point of view. Some people say that it also affects the ability of brake servos, power steering, etc. to operate, but I've always been slightly dubious of this as long as the engine is running.
HellDiver said:
Small 16v engines are most efficient around 3800rpm (before they "come on cam" at 4000rpm). You'll probably find in 5th at 70 is about 3800-4000rpm. So, try and keep the engine around there on load.
Pretty much all 16 valve 4 pot BSFC maps that I've seen peak between 2000-3000 rpm, certainly nothing like as high as 4000rpm, as friction takes over by then.As said, 80% LOAD is usually where the engine is most efficient before it goes into overfuelling, but this usually equates to a lower throttle opening as it's a non linear device.
Probably about 50-70% throttle at low engine speeds is a good sweet spot. The best piece of advice for the OP would be to get a runup and sacrifice speed on the gradient while still applying a decent amount of throttle at not too high an RPM.
The Wookie said:
Pretty much all 16 valve 4 pot BSFC maps that I've seen peak between 2000-3000 rpm, certainly nothing like as high as 4000rpm, as friction takes over by then.
Funny, EVERY 16v engine I've driven has a kick in power at 4000rpm. Most pronounced on the 1.4 K-series, and Vauxhall's 1.8 engine. Even my old 1.8 Duratec doesn't pull especially well until 3800-4000rpm.deveng said:
Theory says low revs high throttle, but it's all down to how the car is mapped.
Not really - Low engine speed combined with high throttle = high engine load = Long injector durationAs others have said, engines are at their most efficient, when the engine speed is low, but the engine is not labouring
The biggest influences on fuel consumption are,
1. Weight - around town where the mass has to be accelerated every time you move off or accelerate
2. Speed - above about 30 mph the energy you need to overcome wind resistance increases as the square of any speed increase, which is a lot by the time you get on the motorway.
Beyond these big things it's hard to achieve much variance in fuel consumption in "normal" driving. Not least because a manual car with 5 or 6 gears and driven sensibly will generally be in the right gear. Modern auto's with 7 or 8 gears can achieve better fuel consumption than manual versions of the same car by keeping the engine in exactly the right rev range for maximum fuel efficiency.
1. Weight - around town where the mass has to be accelerated every time you move off or accelerate
2. Speed - above about 30 mph the energy you need to overcome wind resistance increases as the square of any speed increase, which is a lot by the time you get on the motorway.
Beyond these big things it's hard to achieve much variance in fuel consumption in "normal" driving. Not least because a manual car with 5 or 6 gears and driven sensibly will generally be in the right gear. Modern auto's with 7 or 8 gears can achieve better fuel consumption than manual versions of the same car by keeping the engine in exactly the right rev range for maximum fuel efficiency.
matthewg said:
Yeah so it is best to be in 4th at 40mph rather than 5th at 40mph, in many cases?
My engine feels like it is labouring in 5th at 35-40mph.
This is definitely true in the Elise. 3rd gear in 30 limits, 4th gear in 40 limits, 5th gear in 50 limits... and usually back down to second in national speed limits. My engine feels like it is labouring in 5th at 35-40mph.
Ozzie Osmond said:
Beyond these big things it's hard to achieve much variance in fuel consumption in "normal" driving. Not least because a manual car with 5 or 6 gears and driven sensibly will generally be in the right gear. Modern auto's with 7 or 8 gears can achieve better fuel consumption than manual versions of the same car by keeping the engine in exactly the right rev range for maximum fuel efficiency.
I've never actually come across a case where torque converter automatics are more efficient than manuals. They often beat them in the official tests, but I've never actually known this to be born out in reality unless you drive the manual in the wrong gear all the time. 
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